1. Field of the Invention
The present invention relates to a photomask for aberration measurement, an aberration measurement method, a unit for aberration measurement and a manufacturing method for the unit.
2. Description of the Background Art
In recent years increases in integration and miniaturization of semiconductor integrated circuits have been remarkable. Together with that, miniaturization of circuit patterns formed on a semiconductor substrate (hereinafter referred to simply as wafer) has made rapid progress.
In particular, photolithographic technology is widely recognized as a basic technology used for pattern formation. Accordingly, a variety of developments and improvements have recently been implemented. However, the miniaturization of patterns shows no sign of slowing down and demand for increase in pattern resolution has continued to increase.
This photolithographic technology is a technology wherein patterns on a photomask (original image) are transferred to a photoresist applied to a wafer and wherein the etched film, which is the layer beneath the photoresist, is patterned by using the transcription on the photoresist.
At the time of the transcription onto the photoresist, a development process is carried out on the photoresist in which the type of photoresist wherein the portion struck by light is removed through this development process is called a positive type photoresist while the type of photoresist wherein the portion that has not been struck by light is removed is called a negative type photoresist.
In general, the resolution limit R (nm) in the photolithographic technology using a scale down exposure method is represented as:
R=k1xc2x7xcex/(NA)
wherein xcex is the wavelength (nm) of the utilized light, NA is a numerical aperture in the projection optical system of the lenses and k1 is a constant that depends on the image formation condition and on the resist process.
As can be seen from the above equation, a method of reducing the values of k1 and xcex while increasing the value of NA may be adopted in order to achieve an increase in the resolution limit R, that is to say, in order to gain microscopic patterns. Namely, the constant that depends on the resist process is reduced while a shorter wavelength and a greater NA value are utilized.
Among these, it is technically difficult to shorten the wavelength of the light source and, therefore, it is required to increase the NA value for the same wavelength. The introduction of a greater NA value, however, makes the focal depth xcex4(xcex4=k2xc2x7xcex/(NA)2) of light shallow so that there is a problem wherein deterioration of form and of dimensional precision occurs in the formed patterns.
It is necessary to carry out a pattern design wherein the aberration of the projection lens, and the like, are taken into consideration in order to transcribe the patterns of the photomask to the photoresist with a high precision in the above described photolithographic technology. In order to achieve this, it is necessary to precisely measure the aberration of the projection lens, and the like.
As for a conventional method of measuring the aberration of the projection lens, there is a method shown in the following Reference 1.
Reference 1: H. Nomura et al., xe2x80x9cHigher order aberration measurement with printed patterns under extremely reduced "sgr" illuminationxe2x80x9d, Proc. SPIE Vol. 3679, (1999), pp. 358-367.
In this method aberration is measured by forming a pattern using a photomask as described in the following Reference 2. In the following, this measurement method is described.
Reference 2: H. Nomura et al., xe2x80x9cOverlay Error due to Lens Coma and Asymmetric Illumination Dependence on Pattern Featurexe2x80x9d, Proc. SPIE Vol. 3332, pp. 199-210.
FIGS. 17A, 17B and 17C are views showing the configurations of the pattern of the photomask that is used for the aberration measurement method described in the above reference and the pattern for aberration measurement. In addition, FIGS. 18A to 18F are schematic cross sectional views showing the formation method for the pattern for aberration measurement described in the above Reference 2 in process order.
Referring to FIG. 18A, first, a wafer 121 is prepared wherein a photoresist 121b is applied to a semiconductor substrate 121a. 
Referring to FIG. 18B, the pattern of a photomask 105A, shown in FIG. 17A, is exposed to photoresist 121b of wafer 121. Through this first exposure, photoresist 121b is selectively exposed to light. Here, the portions of photoresist 121b that are exposed to light are shown as white areas while the portions that are not exposed to light are shown as diagonally hatched areas going from the upper left to the lower right.
Referring to FIG. 18C, the pattern of a photomask 105B, shown in FIG. 17B, is further exposed to photoresist 121b. Through this second exposure photoresist 121b is selectively exposed to light. Here, the portions of photoresist 121b that are not exposed to this exposure light are shown as diagonally hatched areas going from the upper right to the lower left. FIG. 18D is an enlarged view showing the region R1 while FIG. 18E is an enlarged view showing the region R2.
After this, photoresist 121b is developed and, then, only the regions (regions shown by cross hatching) that are not exposed to the exposure light through the first and second exposures remain so as to form resist pattern 121b as shown in FIG. 18F. This resist pattern 121b has a form as shown in FIG. 17C as represented in a plane manner.
Here, FIG. 18F corresponds to the cross sectional view along line XVIIIxe2x80x94XVIII of FIG. 17C.
Thus, only a portion of the line and space pattern (L/S pattern) is extracted by eliminating the portions other than the L/S pattern portion through a double exposure according to a conventional method. The relative movement amount between the extracted L/S pattern and the formed pattern of large dimensions is measured with respect to many L/S patterns of which the pitch and the direction differ so that an odd aberration is found from the change in the pitch of this movement amount.
In addition, an even aberration is found from change in the pitch and the direction of the optimal focus after finding the optimal focus from the range of the resolution focus with respect to a large number of L/S patterns of which the pitch and direction differ.
According to this method, however, it is necessary to carry out a very large number of measurements, as shown in FIGS. 4 and 5 of Reference 1, in order to find the lens aberration in projection optical system and there is a problem wherein a great deal of labor and effort are required.
In addition, it is necessary to carry out the exposure by reducing the aperture size of the iris for the detection of the movement amount the optimal focus of the pattern, of which the pitch is large, and there is also a problem wherein a great deal of labor and effort are required for the exposure.
In addition, it is necessary to carry out the exposure by greatly reducing the aperture size of the iris in order to find the aberration in the vicinity of the center part of the iris and there is also a problem wherein this cannot be implemented by means of a commercially available exposure unit.
In addition, a large mask region becomes necessary since a large number of pattern types are used in order to carry out the measurement at a specific field point so that the inside of the field cannot be examined in detail. Concretely, there is a problem wherein the measurement can only be carried out to the degree wherein 8 mm is divided into three portions as shown in FIG. 3 of Reference 1.
It is an object of the present invention to provide a photomask for aberration measurement, an aberration measurement method, a unit for aberration measurement and a manufacturing method for the unit that can measure the aberration of a lens that requires little labor and that ramifies the inside of the field.
A photomask for aberration measurement of the present invention is a photomask for aberration measurement for measuring a lens aberration in the projection optical system of an exposure unit and is provided with a substrate, a plurality of measurement patterns, a light blocking film and a plurality of reference patterns. The substrate allows the exposure light to pass through and has a measurement pattern formation region and a reference pattern formation region. The plurality of measurement patterns are formed in the measurement pattern formation region on the top surface of the substrate. The light blocking film is formed in the measurement pattern formation region on the rear surface of the substrate and has an aperture pattern on the rear surface of the substrate for substantially differentiating the respective incident angles of the exposure light that enters plurality of measurement patterns. The plurality of reference patterns are formed in the reference pattern formation region on the top surface of the substrate. The rear surface of the substrate in the reference pattern formation region is formed so that the respective incident angles of the exposure light that enters the plurality of reference patterns become substantially equal.
According to the photomask for aberration measurement of the present invention, the respective incident angles of the exposure light that enters the plurality of measurement patterns can be substantially differentiated by means of the aperture pattern on the rear surface of the photomask. Thereby, an image of each of the plurality of measurement patterns can be formed on the photosensitive material through light in the diagonal direction. The image of the measurement pattern that is formed through the incidence of light from the diagonal direction causes a positional shift on the surface of the photosensitive material in the case that there is an aberration in the projection lens. Therefore, the aberration of the projection lens can be measured by measuring the amount of positional shift of the transferred pattern that is formed by means of the image of this measurement pattern through the comparison of the transferred pattern with the reference pattern. Since the aberration of the projection lens can be simply measured in such a manner, measurement of the aberration requires less labor than in the prior art.
In addition, since the aberration can be measured for each of the measurement pattern formation regions, it is possible to measure the aberration with respect to a plurality of points within the field. Therefore, the distribution of the aberrations within the field can be comprehended in detail.
In the above described photomask for aberration measurement, xc2xd of the diameter of the aperture pattern on the rear surface of the photomask preferably satisfies sin(xcfx86)xe2x89xa6(NA/M)/5 wherein the scale down projection ratio is M, the numerical aperture is NA and the half angle of the angle of viewing the aperture pattern on the rear surface of the photomask from the point on the top surface of the substrate that is directly opposite to the center position of the aperture pattern on the rear surface of the photomask is xcfx86.
In the case that sin(xcfx86) exceeds (NA/M)/5 of the above description, the angle range xcfx86 of the illumination light becomes too wide so that it becomes difficult to detect the local characteristics of the iris.
In the above described photomask for aberration measurement, xc2xd of the dimension of each of the plurality of measurement patterns preferably satisfies sin(xcex6)xe2x89xa6(NA/M)/5 wherein the scale down projection ratio is M, the numerical aperture is NA and the half angle of the angle of viewing the above measurement pattern from the point on the rear surface of the substrate that is directly opposite to the center position of the measurement pattern is xcex6.
In the case that sin(xcex6) exceeds (NA/M)/5 of the above description, the dimension of the measurement pattern becomes too large so that the range of the iris that contributes to the image formation of the pattern becomes too large and the resolution within the iris in the phase difference measurement is lowered and the precision of the measurement of the aberration deteriorates.
In the above described photomask for aberration measurement, at least some of the plurality of measurement patterns are preferably positioned in the range of the viewing angle 2xcex1 that satisfies sin(xcex1)xe2x89xa6(NA/M)xc3x97"sgr" having the point on the top surface of the above substrate directly opposite to the center position of the aperture pattern on the rear surface of the photomask as the center wherein the scale down projection ratio is M, the numerical aperture is NA and the ratio of sine of the half angle of the spread angle of the illumination with which the rear surface of the mask is irradiated to sine of maximum incident angle of the conversion rays in the projection optical system, that is to say, NA is "sgr".
The portion within the above described range is illuminated from the aperture pattern on the rear surface of the photomask and, therefore, a measurement pattern can be illuminated in the case that this measurement pattern is positioned within this range.
The center position of each pattern of the plurality of reference patterns preferably has the same arrangement as the center position of each pattern of the plurality of measurement patterns in the above described photomask for aberration measurement.
Thereby, each of the plurality of reference patterns and each of the plurality of measurement patterns can be effectively overlapped through a shifted double exposure.
In the above described photomask for aberration measurement, a light blocking film preferably has apertures with a constant aperture ratio in the region of the rear surface of the substrate wherein the viewing angle xcex2 made as viewed from the top surface of the substrate satisfies sin(xcex2)xe2x89xa7(NA/M)xc3x97"sgr" having the point on the rear surface of the substrate that is directly opposite to an arbitrary point in the region wherein the plurality of reference patterns are arranged as the center point wherein the scale down projection ratio is M and the numerical aperture is NA.
Thereby, the photomask can be formed so that the respective incident angles of the exposure light that enters the plurality of reference patterns become substantially equal.
In the above described photomask for aberration measurement, the aperture pattern formed in the measurement pattern formation region on the rear surface of the substrate for substantially differentiating the respective incident angles of the exposure light that enters the plurality of measurement patterns is preferably circular.
In the above described photomask for aberration measurement, the external form of each of the plurality of measurement patterns is preferably square.
A box pattern can, thus, be used as the measurement pattern.
In the above described photomask for aberration measurement, the external form of each of the plurality of reference patterns is preferably square.
A box pattern can, thus, be used as the reference pattern.
In the above described photomask for aberration measurement, each of the plurality of measurement patterns is preferably arranged on the dot that forms a tetragonal lattice wherein the viewing angle xcex4 of xc2xd of the pitch P of the points that form the tetragonal lattice as viewed from the points on the rear surface side of the substrate that are directly opposite to the points that form the tetragonal lattice satisfies sin(xcex4)xe2x89xa6(NA/M)/5 wherein the scale down projection ratio is M and the numerical aperture is NA.
Thereby, plurality of aperture patterns 2a for measurement can be arranged in a concentrated manner so that it becomes possible to measure the phase distribution within the iris at a high resolution, that is to say, it becomes possible to measure the aberration with a high precision.
In the above described photomask for aberration measurement, the pitch P of the points that form the tetragonal lattice preferably satisfies P/Mxe2x89xa720 xcexcm wherein the scale down projection ratio is M.
In the case that P/M is smaller than 20 xcexcm, the overlap inspection unit cannot recognize the transferred patterns that correspond to two adjacent aperture patterns 2a for measurement as differing patterns.
In the above described photomask for aberration measurement, the dimension Ib1 of the square satisfies Ib1/Mxe2x89xa75 xcexcm wherein the scale down projection ratio is M.
In the case that the dimension Ib1 of the measurement pattern is small to the degree that the above condition is not satisfied, the overlap inspection unit cannot measure the transferred pattern of the measurement pattern.
In the above described photomask for aberration measurement, the dimension Ib2 of the square satisfies Ib2/Mxe2x89xa75 xcexcm wherein the scale down projection ratio is M.
In the case that the dimension Ib2 of the reference pattern is small to the degree that the above condition is not satisfied, the overlap inspection unit cannot measure the transferred pattern of the reference pattern.
In the above described photomask for aberration measurement, preferably, either one of the measurement pattern or the reference pattern corresponds to an inner box pattern of a box-in-box type mark while the other of the measurement pattern or the reference pattern corresponds to an outer box pattern.
Thereby, the amount of positional shift can be easily measured.
An aberration measurement method according to the present invention is provided with the step of transferring a plurality of measurement patterns of the above described photomask for aberration measurement onto a photosensitive material, the step of measuring the amount of positional shift of the transferred patterns and the step of calculating the inclination of the equiphase wave surface that is theoretically proportional to the amount of positional phase so as to find the wave aberration by using the information of the calculated inclination of the equiphase wave surface.
According to the aberration measurement method of the present invention, the respective incident angles of the exposure light to the plurality of measurement patterns can be made to differ substantially by means of an aperture patterns on the rear surface of the photomask. Thereby, an image of each of the plurality of measurement patterns can be formed by light in the diagonal direction relative to the photosensitive material. An image of a measurement pattern that is formed by light that enters from the diagonal direction is formed in a shifted position on the surface of the photosensitive material in the case that there is an aberration in the projection lens. Therefore, the amount of positional shift of the transferred pattern formed through the image formation of this measurement pattern can be measured in comparison with the transferred pattern of the reference pattern and, thereby, the aberration of the projection lens can be measured. The aberration of the projection lens can easily be measured in the above manner and, therefore, the aberration can be measured requiring less labor than in the prior art.
In addition, the aberration can be measured for each of the plurality of measurement pattern formation regions and, therefore, measurements of lens aberrations in a projection optical system are possible with respect to a large number of points within the field. Accordingly, the distribution of the lens aberration in the projection optical system can be found in detail within the field.
In the above described aberration measurement method, the step of measuring the amount of positional shift of the transferred pattern has the step of measuring the amount of mutual positional shift between the transferred pattern of the measurement pattern that has been transferred to the photosensitive material through either one of the first or second exposure and the transferred pattern of the reference pattern that has been transferred to the photosensitive material through the other of the first or second exposure by carrying out the second exposure with the photosensitive material being shifted relative to the photomask for aberration measurement after carrying out the first exposure by using the photomask for aberration measurement.
By carrying out a double exposure in such a manner, the amount of mutual positional shift between the transferred pattern of the measurement pattern and the transferred pattern of the reference pattern can be easily measured.
In the above described aberration measurement method, the step of measuring the amount of positional shift of the transferred pattern preferably has the step of measuring the amount of positional shift of the transferred pattern relative to the standard of a coordinate measurement unit by using the coordinate measurement unit.
Thereby, limitations of the form of the pattern for measuring the positional shift are reduced so that the freedom of formation design becomes high.
In the above described aberration measurement method, the coordinate measurement unit is preferably a projection exposure unit.
Thereby, the limitation of the formation of the pattern for measuring the positional shift is reduced so that the freedom of the formation design becomes high and it becomes unnecessary to separately prepare the coordination measurement unit.
In the above described aberration measurement method, preferably either one of the transferred pattern of the measurement pattern and the transferred pattern of the reference pattern is an inner box pattern of the box-in-box type mark while the other is an outer box pattern, wherein the amount of positional shift between the inner box pattern and the outer box pattern is measured by an overlap inspection unit.
Thereby, measurement of the amount of positional shift of overlapping becomes easy.
The above described aberration measurement method is preferably further provided with the steps of measuring the phase distribution within the iris of each focus when a plurality of measurement patterns are transferred to a photosensitive material with the focus being varied, of calculating the phase distribution within the iris at the time of defocusing with the standard of the phase distribution within the iris at the time of the optimal focus and of specifying the position of the iris center by finding the center of the paraboloid of revolution that represents the change of the phase distribution within the iris through defocusing.
Thereby, the positional shift between the aperture pattern on the rear surface of the photomask and the measurement pattern can be corrected so that a lens aberration measurement in the projection optical system with a high precision becomes possible.
A unit for aberration measurement of the present invention is a unit for aberration measurement that is provided with a photomask for aberration measurement for measuring a lens aberration in a projection optical system of an exposure unit, which is provided with a photomask for aberration measurement on which patterns are formed, an illumination optical system for irradiating the photomask for aberration measurement with exposure light and a projection optical system for projecting an image of the patterns of the photomask for aberration measurement onto a photosensitive material. The photomask for aberration measurement is provided with a substrate, plurality of measurement patterns, a light blocking film and a plurality of reference patterns. The plurality of measurement patterns is formed on the top surface of the substrate in a measurement pattern formation region. The light blocking film is formed on the rear surface of the substrate in a measurement pattern formation region and has a rear surface aperture pattern for substantially differentiating the respective incident angles of the exposure light that enters the plurality of measurement patterns. The plurality of reference patterns is formed on the top surface of the substrate in a reference pattern formation region. The rear surface of the substrate in the reference pattern formation region is formed so that the respective incident angles of the exposure light that enters the plurality of reference patterns become substantially the same.
According to the unit for aberration measurement of the present invention, the respective incident angles of the exposure light that enters each of the plurality of measurement patterns can be substantially differentiated by means of the rear surface aperture pattern. Thereby, an image of each of the plurality of measurement patterns can be formed by light in the diagonal direction relative to the photosensitive material. The image of the measurement pattern that is formed by light, which enters from the diagonal direction, is formed in a shifted position on the surface of the photosensitive material in the case that there is an aberration in the projection lens. Accordingly, the amount of positional shift of the transferred pattern formed through the image formation of this measurement pattern is measured as compared to the transferred pattern of the reference pattern and, thereby, the aberration of the projection lens can be measured. The aberration of the projection lens can be simply measured in such a manner and, therefore, the aberration can be measured with less labor than in the prior art.
In addition, since an aberration can be measured for each of the plurality of the measurement pattern regions, measurements of aberrations are possible at a large number of points within the field. Accordingly, the aberration distribution within the field can be found in detail.
A manufacturing method of a device according to the present invention uses the above described aberration measurement method.
Thereby, the distribution of the lens aberration within the field can be measured in detail while the measurement requires little labor and, therefore, the patterns that can be formed with a high precision can be known in advance so that the patterns of the device can be formed by exclusively using such patterns.
In the above described manufacturing method of a device, the device that is formed by using the aberration measurement method is a semiconductor device.
Though the above described manufacturing method of a device is suitable for the manufacture of devices (electronic devices) such as a thin film magnetic head or a liquid crystal display element, it is also suitable for the manufacture of a semiconductor device.